Tuned vibration absorption devices are usually tuned mass dampers comprising predetermined sets of passive masses and passive damped springs to reduce the susceptibility to vibration of a structure. In these pre-tuned passive vibration absorption devices, the operational frequency (that is, the resonance frequency) of the devices remains unchanged once the devices are designed and fabricated.
In the prior art, tunable vibration absorption devices are typically tunable mass dampers (or adjustable tuned mass dampers) comprising sets of passive masses and adjustable damped springs such that tuning of the operational frequency of the devices is achieved manually by adjusting the stiffness of the springs through some mechanical means.
Advanced tunable vibration absorption devices are generally fabricated using smart materials (also called active, intelligent or adaptive materials) as the principal tuning means to provide the distinct features of tunable stiffness via an external tuning signal. With such tuning means, one may easily and precisely adjust the operational or resonance frequency of such devices to optimally match the targeted resonance frequency of the corresponding under-damped vibrating structures so that an additional damping can be properly introduced to the vibrating structures.
Examples of smart materials mainly include piezoelectric and magnetostrictive materials. The observed tunable stiffness (and hence the operational frequency) of the smart material-based tunable vibration absorption devices originate from an external field-induced characteristic property change of the smart material elements (that is, their stiffness) as well as the cooperative property change of the devices as a whole. Hence, the tunability and controllability of the stiffness (and hence the operational frequency) of the devices play an important factor in absorbing or damping vibrational energy.
If the vibration absorption devices are not tunable or if the tunability and controllability of the stiffness of the tunable vibration absorption devices are not sufficiently high such that the stiffness of the devices cannot be properly optimized for the vibrating structures, installing such devices onto the vibrating structures may result in the direct transmission of vibrational energy from the structures to the devices without experiencing any energy absorption or damping effect. In the worst cases, the vibration level of the vibrating structures may further be enhanced. Consequently, the advantages of deploying tunable vibration absorption devices, in particular smart material-based tunable vibration absorption devices, onto vibrating structures are two-fold: first, their nature of tunable stiffness enables their operational frequency to easily and optimally match the targeted resonance frequency of under-damped vibrating structures without adding any external mass; second, their nature of having a reasonably high damping ratio enables a wider range structural applications without adding any external or extra absorption or damping means, such as damped springs.
Nonetheless, while smart material-based tunable vibration absorption devices are more effective and useful as compared with traditional tuned and tunable (or adjustable tuned) vibration absorption devices (i.e., tuned and tunable (or adjustable tuned) mass dampers), these state-of-the-art devices typically suffer from several shortcomings. For example, they acre only provided with a vibrational energy absorption function through external tuning of the characteristic properties of the smart material elements. Hence, they are only limited to a semi-active (or an open-loop) mode of operation, where a predetermined or a manually-tuned input signal is applied to the smart material elements of the devices without the assistance of any automatic control system. If an active (or a closed-loop) mode of operation is necessary, at least one separate sensor (such as an accelerometer or a force sensor) is required to gather the so-called “predetermined” or “manual-tuned” input signal. Thus, separate sensors have to be installed together with the devices. It is clear that measurement cost and complexity will inevitably increase. Another shortcoming of this approach is that, in practice, it is quite difficult to reliably co-locate both the devices and separate sensors. Accordingly, discrepancy between a sensor output signal and an actual structural vibration may occur.
Prior art tunable vibration absorption devices can generally only provide uni-directional tuning of the operational frequency (which is related to the stiffness). In particular, they work well in a high frequency regime as the frequency tuning elevates from a specific frequency value defined by an off-state natural frequency of the devices. Besides the characteristic property (such as stiffness) of the smart material elements in the devices, the operational frequency of these devices depends heavily on the whole structure of the devices. Thus, state-of-the-art designs have to incorporate a number of structural components so as to provide a less sensitive mounting means for connection with the vibrating structures.
A prior art tunable vibration absorption device is described in U.S. Pat. No. 6,681,908 for an “Adjustable Tuned Mass Damper”. This invention teaches a tuned mass damper which is tunable by manually adjusting a spring stiffness of the damper through a screw connected to the spring. Rotating the screw changes the spring stiffness and thereby the natural resonance frequency of the mass and spring combination. Although this design is quite simple, manual and mechanical tunings make the damper difficult to integrate with electrical or automated control systems. If the design is to be modified for electrical or automated tuning, the patent suggests inclusion of a separate accelerometer. As explained above, this will, amongst other things, increase the cost and complexity of the damper.
Another prior art tunable vibration absorption device is described in U.S. Patent Publication No. 2002/0060268 entitled, “Method and Apparatus for Improved Vibration Isolation”. It teaches a vibration isolator that is tunable through electrically adjusting the motion of an enclosed fluid mass through a piezoelectric actuation pump so as to cancel a frequency of oscillatory forces from a vibrating structure. This design of vibration isolator is undesirably complex to manufacture on an industrial scale and its reliance on adjusting the motion of a fluid mass to counter external oscillatory forces makes it difficult to monitor the electrical or automated tuning in practice.